Sulfur crosslinking bonds are introduced in a vulcanized rubber, and mono-, di-, tri-, poly-sulfur linkages and the like are introduced depending on the length of the sulfur crosslinking bonds. The length of such sulfur crosslinking bonds greatly affects the physical/chemical properties of the vulcanized rubber. For example, the shorter the length of the crosslinking bonds, the greater the rigidity of the vulcanized rubber. Therefore, it is very important to analyze the length of sulfur crosslinking bonds in a vulcanized rubber.
However, a method for analyzing the length of sulfur crosslinking bonds in the vulcanized rubber is extremely limited. For example, when analyzed by NMR, it is only possible to analyze mono-sulfur linkages (sulfur crosslinking bonds composed of one sulfur atom) and poly-sulfur crosslinking bonds (sulfur crosslinking bonds composed of two or more sulfur atoms), and it is difficult to analyze an average sulfur length.
In addition to NMR, it is possible to distinguish the types of crosslinking bonds by vulcanization using a solvent method. However, it can only analyze mono-sulfur linkages (sulfur crosslinking bonds composed of one sulfur atom), di-sulfur linkages (sulfur crosslinking bonds composed of two sulfur atoms) and poly-sulfur linkage (sulfur crosslinking bonds composed of three or more sulfur atoms), and it is also difficult to analyze the average sulfur length. In addition, the solvent method must use a toxic solvent and has a long measurement time, which are disadvantages.
As such, the reason why it is difficult to analyze the average sulfur length of sulfur crosslinking bonds in the vulcanized rubber is that the chemical shift between the structures of the di-sulfur linkages (sulfur crosslinking bonds composed of two sulfur atoms) and the poly-sulfur linkages (sulfur crosslinking bonds composed of three or more sulfur atoms) is not so significant, only enabling for the mono-sulfur linkages of the poly-sulfur linkages to be distinguished. Also, in the case of the poly-sulfur linkages, it is difficult to confirm how many sulfur atoms participate in the crosslinking.
On the other hand, since the integrated intensity of a signal in an NMR spectrum is proportional to the nuclear species appearing in the corresponding signal, quantification can be performed based on the specific signal of the compound to be quantified. That is, if a standard substance that knows its molecular structure is added together to a sample for an accurate quantitative analysis, the integrated intensity of a signal of a standard substance in a NMR spectrum, and the integrated intensity of a specific signal of a compound to be quantified can be obtained. In this case, since the molecular structure and the amount of the standard substance are known, quantitative analysis of a compound in the sample is possible. The standard substance that is added together with the sample is referred to as an ‘internal standard’.
However, the quantitative analysis described above has a problem that it is difficult to apply to an insoluble sample, particularly to the vulcanized rubber which is the target for analysis of the present invention. The most important reason is that in order to obtain a significant NMR spectrum, the sample and the standard substance should be uniformly mixed, but in the case of the insoluble sample, the sample is not mixed in a solvent and therefore, it is difficult to uniformly mix the sample.
In this regard, in the present invention, in order to analyze the length of the sulfur crosslinking bonds in the vulcanized rubber, the NMR spectra of the standard substance and the vulcanized rubber are separately obtained and analyzed, rather than mixing the standard substance with the vulcanized rubber, and the standard substance used herein is referred to as an ‘external standard’. Further, the length of sulfur crosslinking bonds in the vulcanized rubber was analyzed by extracting useful information from each NMR spectrum.